Room temperature-curable compositions based on polyoxyalkylene organic polymers having a reactive silicon containing group are not only inexpensive but also have various meritorious properties making them suitable for use as, inter alia, architectural sealants (called modified silicone sealants). While these are required to be satisfactory enough in various properties, weather resistance over a long period of time is no less important than mechanical properties, such as modulus, elongation at break, strength at break, etc., and many studies were undertaken in these respects. It has been found that, as far as mechanical properties such as modulus, elongation at break, and strength at break are concerned, addition of a phthalic ester or a polyether plasticizer results in the expression of characteristics suitable for sealing materials. With regard to weather resistance, it is disclosed in, inter alia, Japanese Kokai Publication Sho-59-122541 and Sho-60-031556 that weather resistance can be improved by blending a vinyl polymer having a reactive silicon containing group with a polyoxyalkylene organic polymer having a reactive silicon containing group as compared with the exclusive use of such a polyoxyalkylene organic polymer having a reactive silicon containing group alone.
For the purpose of obtaining a curable composition providing satisfactory mechanical properties and weather resistance over a long period of time, the inventors of the present invention prepared a curable composition by adding a phthalic ester or polyether plasticizer to a blend of a vinyl polymer having a reactive silicon containing group and a polyoxyalkylene organic polymer having a reactive silicon containing group on the basis of the above known technologies, investigated the composition, and found that whereas mechanical properties were improved in fact, there was still room for improvement in weather resistance.